1. Field of the Invention
The invention pertains generally to optical systems, e.g., optical fiber communication systems, which include one or more wavelength-tunable semiconductor lasers.
2. Background of the Invention
Wavelength-tunable semiconductor lasers are considered desirable for use in a number of different types of optical systems. For example, optical fiber communication systems employing wavelength division multiplexing (WDM) necessarily include a plurality of light sources, typically semiconductor lasers, which emit optical signals at different wavelengths. In addition, it has been proposed that the stations of local area networks (LANs) communicating via optical fiber buses employ semiconductor lasers which emit different-wavelength laser radiation, the different wavelengths serving to identify the stations. The different wavelengths employed in these optical systems can be achieved by, for example, using semiconductor lasers having active layers with different material compositions. However, the fabrication of such lasers introduces undesirable complexity into the manufacturing process. By contrast, such complexity is avoided by using semiconductor lasers which are compositionally identical, but wavelength-tunable, hence the desirablility of tunable lasers.
Distributed feedback (DFB) and distributed Bragg reflector (DBR) semiconductor lasers are wavelength-tunable, and thus useful in the above-described optical systems. However, the maximum reported tuning range of, for example, DBR semiconductor lasers is only 116 Angstroms. (See Bjorn Brobert et al, "Widely Tunable Bragg Reflector Integrated Lasers in InGaAsP-InP," Applied Physics Letters, 52(16), Apr. 18 1988, pp. 1285-1287.) As a result, the number of different wavelength signals which can be employed in a WDM optical system, as well as the number of stations in a LAN emitting signals at different wavelengths, is limited. Consequently, tunable semiconductor lasers having tuning ranges greater than 116 Angstroms have been sought.
A new semiconductor laser which, it was hoped, would be wavelength-tunable includes an active layer which consists of a so-called doping superlattice, also called a n-i-p-i crystal. That is, the active layer is crystalline in nature, and has a composition which typically includes III-V compound semiconductor material, e.g., GaAs. In addition, the active layer includes periodically spaced sheets (layers) of n-type and p-type dopant, having delta function-like doping profiles (in the direction of the growth axis of the active layer), separated by intrinsic semiconductor material. The presence of the dopant sheets produces a sawtooth-shaped modulation of the energy band diagram associated with the intrinsic semiconductor material. (Regarding doping superlattices see, e.g., G. H. Dohler, "Doping Superlattices ("n-i-p-i Crystals")," IEEE Journal of Quantum Electronics, Vol. QE-22, No. 9, Sept. 1986, pp. 1682-1694.)
Significantly, semiconductor lasers employing doping superlattice active layers exhibit tunable, spontaneous emission. That is, when such a laser is (uniformly) optically or electrically pumped, at levels which are below those needed to achieve stimulated emission, it is believed that the resulting electron-hole pairs produced in the intrinsic semiconductor material of the active layer serve to screen (compensate) the dopants in the dopant sheet and, as a result, reduce the degree of band diagram modulation produced by the dopant sheets. Moreover, the greater the intensity of the pumping excitation, the greater is the screening effect. As a consequence, by varying the intensity of the pumping excitation, the wavelength of the spontaneous emission is readily changed. However, artisans working in the field have asserted that at the threshold levels needed to achieve stimulated emission, the density of electron-hole pairs becomes so high that the band diagram modulation produced by the dopant sheets is completely eliminated, i.e., the valence and conduction bands are completely flat. Consequently, it has been concluded that the wavelength of the stimulated emission cannot be tuned. (See E. F. Schubert et al, "GaAs Sawtooth Superlattice Laser Emitting at Wavelengths .lambda.&gt;0.9 .mu.m," Applied Physics Letters, 47(3), Aug. 1, 1985, pp. 219-221; and B. A. Vojak et al, "Photopumped Laser Operation of GaAs Doping Superlattices," Applied Physics Letters, Vol. 48, No. 3, Jan. 20, 1986, pp. 251-253.)
Thus, those engaged in developing optical systems employing wavelength-tunable semiconductor lasers have sought, thus far without success, tunable semiconductor lasers having a wavelength tuning range greater than 116 Angstroms.